Re: [BUG] cgroupv2/blk: inconsistent I/O behavior in Cgroup v2 with set device wbps and wiops

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+Cc Kuai

On Mon, Aug 12, 2024 at 11:00:30PM GMT, Lance Yang <ioworker0@xxxxxxxxx> wrote:
> Hi all,
> 
> I've run into a problem with Cgroup v2 where it doesn't seem to correctly limit
> I/O operations when I set both wbps and wiops for a device. However, if I only
> set wbps, then everything works as expected.
> 
> To reproduce the problem, we can follow these command-based steps:
> 
> 1. **System Information:**
>    - Kernel Version and OS Release:
>      ```
>      $ uname -r
>      6.10.0-rc5+
> 
>      $ cat /etc/os-release
>      PRETTY_NAME="Ubuntu 24.04 LTS"
>      NAME="Ubuntu"
>      VERSION_ID="24.04"
>      VERSION="24.04 LTS (Noble Numbat)"
>      VERSION_CODENAME=noble
>      ID=ubuntu
>      ID_LIKE=debian
>      HOME_URL="https://www.ubuntu.com/";
>      SUPPORT_URL="https://help.ubuntu.com/";
>      BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/";
>      PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy";
>      UBUNTU_CODENAME=noble
>      LOGO=ubuntu-logo
>      ```
> 
> 2. **Device Information and Settings:**
>    - List Block Devices and Scheduler:
>      ```
>      $ lsblk
>      NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
>      sda     8:0    0   4.4T  0 disk
>      └─sda1  8:1    0   4.4T  0 part /data
>      ...
> 
>      $ cat /sys/block/sda/queue/scheduler
>      none [mq-deadline] kyber bfq
> 
>      $ cat /sys/block/sda/queue/rotational
>      1
>      ```
> 
> 3. **Reproducing the problem:**
>    - Navigate to the cgroup v2 filesystem and configure I/O settings:
>      ```
>      $ cd /sys/fs/cgroup/
>      $ stat -fc %T /sys/fs/cgroup
>      cgroup2fs
>      $ mkdir test
>      $ echo "8:0 wbps=10485760 wiops=100000" > io.max
>      ```
>      In this setup:
>      wbps=10485760 sets the write bytes per second limit to 10 MB/s.
>      wiops=100000 sets the write I/O operations per second limit to 100,000.
> 
>    - Add process to the cgroup and verify:
>      ```
>      $ echo $$ > cgroup.procs
>      $ cat cgroup.procs
>      3826771
>      3828513
>      $ ps -ef|grep 3826771
>      root     3826771 3826768  0 22:04 pts/1    00:00:00 -bash
>      root     3828761 3826771  0 22:06 pts/1    00:00:00 ps -ef
>      root     3828762 3826771  0 22:06 pts/1    00:00:00 grep --color=auto 3826771
>      ```
> 
>    - Observe I/O performance using `dd` commands and `iostat`:
>      ```
>      $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
>      $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
>      ```
>      ```
>      $ iostat -d 1 -h -y -p sda
>      
> 	   tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>      7.00         0.0k         1.3M         0.0k       0.0k       1.3M       0.0k sda
>      7.00         0.0k         1.3M         0.0k       0.0k       1.3M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>      5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda
>      5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>     21.00         0.0k         1.4M         0.0k       0.0k       1.4M       0.0k sda
>     21.00         0.0k         1.4M         0.0k       0.0k       1.4M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>      5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda
>      5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>      5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda
>      5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>   1848.00         0.0k       448.1M         0.0k       0.0k     448.1M       0.0k sda
>   1848.00         0.0k       448.1M         0.0k       0.0k     448.1M       0.0k sda1
>      ```
> Initially, the write speed is slow (<2MB/s) then suddenly bursts to several
> hundreds of MB/s.

What it would be on average?
IOW how long would the whole operation in throttled cgroup take?

> 
>    - Testing with wiops set to max:
>      ```
>      echo "8:0 wbps=10485760 wiops=max" > io.max
>      $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
>      $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
>      ```
>      ```
>      $ iostat -d 1 -h -y -p sda
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>     48.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
>     48.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>     40.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
>     40.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>     41.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
>     41.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>     46.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
>     46.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1
> 
> 
>       tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
>     55.00         0.0k        10.2M         0.0k       0.0k      10.2M       0.0k sda
>     55.00         0.0k        10.2M         0.0k       0.0k      10.2M       0.0k sda1
>      ```
> The iostat output shows the write operations as stabilizing at around 10 MB/s,
> which aligns with the defined limit of 10 MB/s. After setting wiops to max, the
> I/O limits appear to work as expected. 
> 
> 
> Thanks,
> Lance

Thanks for the report Lance. Is this something you started seeing after
a kernel update or switch to cgroup v2? (Or you simply noticed with this
setup only?)


Michal

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